Apparatus for electrolytically determining the thickness of metal coatings



` Dec. 28, 1948. W, 5 HERBERT [-1- AL 2,457,234

APPARA s FOR ELEGTROLYTICALLY DETERMINING T THICKNEss oF METAL'COATINGSFiled sept. 25, 1945 Hon/Rn 7,' Kenn/cls.

Patented Dec. 28, 1948 APPARATUS FOR ELECTROLYTICALLY DE- TERMHNING THETHICKNESS F METAL- CATINGS William S. Herbert, Madison, Wis., and HowardT. Francis, Chicago, Ill., assignors to Armour Research Foundation ofIllinois Institute of Technology, a corporation of Illinois ApplicationSeptember 25, 1943, Serial No. 503,828

This invention relates to a method of and to an apparatus fordetermining the thickness of metal coatings on foundations of dissimilarmetals or alloys, or of a nonrnetalllc character.

' Previous methods that have been generally used for measuring thethickness of metallic coatings include the following:

1. Magnetic or electromagnetic method.

2. Jet test and spot test. v

3. Dropping test, and immersion test.

4. Chemical or electrolytic stripping and weighing, or analysis. Y

5. Electrochemical stripping of a smallarea at constant current.

6. Microscopic method.

7. Chord method.

All of the above tests are fully described in the literature and can befound adequately discussed in the Plating and Finishing Guide Book,

(1945), published by Metal Industries Publishing very accurate results.The rst methodis applic-` able only to certain combinations of coatingand base metal (by which term is meant the foundation of metal), and inmany cases does not give very reliable results for thin coatings. Thesecond and third methods are subject to many variables and require closetemperature control for accurate results. The fourth method isapplicable only to determining the averagethickness for fairly largeareas. It is less accurate when used for thinner coatings. The iifthmethod has been used for determining the thickness of chromium platingand is inherently quite accurate, but it has the disadvantage that theend-point is not so sharp as desirable and that the apparatus is'neitherprotable nor simple to operate. microscopic method requires expensiveequipment and a skilled operator and also i-s not applicable to coatingsof soft metal. The chord method is not very accurate and can be appliedto flat surfaces only with diiiiculty.

It is therefore evident that with the greatly expanded production ofplated metal strip, there is an urgent need for a simple and accuratemethod and device for testing and controlling the thickness of the metalcoatings.

We have found that the thickness of metal coatings can be rapidly andaccuratelydetermined by making a small area of the coating the anode inan electrolytic or voltaic cell and then discharging the cell atconstant current until the metal coating is completely removed.According to Faradays law, the current-timeproduct The 2 Claims. (Cl.2011-195) multiplied by the proper factor, givesthe weight of metalremoved. Since the metal is removed from a deiinite area, the thicknessof the metal deposit can be calculatedby the formula w t*fixa in which tis the thickness, w the weight of metal removed, d the density of themetal and a the area. The instrument can also be standardized byapplying the cell to coatings of known" thickness.`

The latter method is useful when the metal coating does not disolve at%eiciency according to Faradays law, but does dissolve in` areproducible manner.

It is therefore an important Jobject of this in` vention to provide amethod whereby the thickness of metallic coatings can be determinedrapidly and with high accuracy.

It is a further important'object of this invention to provide a methodfor determining the thickness of thepart of'metallic coatings unalloyedto the base or foundation metal,` and thus to ascertain the thickness ofboth the alloyed and unalloyed portions of the coating.

It is a further important object oi this invention to provide a simpleand convenient apparatus for carrying out the method hereindescribed'for measuring the thickness of metallic coatings.

It is further important object of this invention to provide a device foruse in determining the thickness of metal coatings that is readilyport-l` tion, illustrated in operative position with respectv to aportion of `a metal coating the' thickness of which is to be determined.

Figure 2 is a wiring diagram illustrative` of the apparatus and methodused in accordance with this invention for determining the thickness ofmetal coatings.

Figure 3 is a greatly enlarged fragmentary sec-` tional view of thedevice illustrated in Figure 1. As shown on the drawings: The referencenumeral It indicates generally a wwall or sheet, having a base orfoundation metal Hf anda coating l2' on one surface thereof. v Thethickness of the coating I2 maybe measured by linals of the apparatusabout to be described, in accordance with the method of our invention.

The apparatus of our invention comprises what may be termed a voltaiccell, which is used as a stripping cell ,for removing the metal coatingover 'a predetermined -area thereof. Said stripping 'zell, which isindicated generally by the reference numeral I3, comprises a cylindricalmetallic meriber I4, having an axial cavity I that is cylindrical inshape and that "is restricted at its upper end by an inturned annularshoulder I6. The cylindrical member I4 is adapted to be positioned uponthe coating I2 and -to rest thereon by gravity to form good electricalContact therewith.

Within the cavity I5 is positioned a composite insulating and sealingmember I'I', which functions as a piston and comprises a cylinder IB ofplastic material, rubber or the like, in the lower end of which isfixedly positioned a sealing washer or gasket I8. Said sealing washer I9extends beyond the lower edge of the cylinder I8 to restuponysaid,coating I 2. The sealing washer I'9als`o 4extends'inwardly of theinner cylindrical surface of said wall I8, as at 2I, to provide anannular shoulder against which rests the lower end of a hollow electrode22. Said hollow electrode 22 is thus insulated by means of said sealingwasher I9 from the metal vcoating I2.

A coiledspring 23 yis positioned about said hollow electrode-22, withits lower end resting against the upperend of s aid cylinder I8 andcentered thereon by a reduced cylindrical portion I8a. The upper end of,s aid spring 23 bears against the intur'ned annular shoulder I6. Saidspring 23 is thus held under pressure between said annularshouldernivand-lthe piston-like member I I to urge the sealing washer I9into sealing contact with said coating I2. v

`Aneiectrolyte 24 is adapted to be introduced into Athe interiorwofwtheh ollow electrode 22 to partiallyllfthe vsamemSaid electrolyte 24 isconfined by means of the lsealing washer I9 to contactlsaidy coating I 2over a predetermined area thereof only. The spring urged` sealing washerI9 prevents the lateral spreading of the electrolyte over the "surfaceof fthe-coating I2.

As illustrated in Figure V2, the stripping cell I3 may begcc'irinacteduanexternal circuit, including a variable resistance an ammeter 25, and aswitch 21. One'of the Wires 28 of the circuit, leads frointhe negativeside of the cell I3, constituted'by al terminal 29 secured in themetallic cylinder I4. The other wire, 30, leads fromthe positive sidelof the cell I3, i. e. the hollow electrode y22.l v.Said electrode 22,thus, serves asthe cathode 'in the cell, lwhile theV metal coating I2,itself, serves"as'th`e anode, and is in direct electrical vcontact withthe cylindrical member I4. When the switch 2l is closed, `the action ofthe electrolyte 24 upon the electrode 22 and coating I2 sets up anelectric current which flows through the external circuit'represented bythewires 28 a1`d30.

The operation of the device just described is as follows:

The metal coated article I0 to be tested is lmade the anode inthestripping cell I3, and a suicient amount of a suitable electrolyte 24is introduced into the hollow cathode 22. The rheostat 26 is adjusted toa predetermined point, at which the proper amount of current will bedrawn from the cell. Then the circuit is closed by the switch 2'I Vandsimultaneously a stopwatch is started. During theA test lit willprobably benecessaryjto adjust the current to a constant value by meansof the rheostat 26, the value of the current being indicated on theammeter 25, which may be a milli-ammeter or a micro-ammeter. Theendpoint, or the removal of all of the coating metal I2, is shown whenthe eurent changes rapidly and can no longer be adjusted to the constantvalue by the rheostat. At this point the stopwatch is stopped and theswitch 2l opened to turn off the current of the cell. The time required,when multiplied by a factor determined for the particular instrument,will give the thickness or Weight of the coating. This factor dependsupon the electrochemical equivalent and density of the coating metal,the current used and corrections for any inaccuracy in the currentmeter, the dimensions required for expressing the thickness, and thearea of the metal coating which has been stripped.

Within certain limits it is possible to adjust the current and area ofcoating stripped so that a measurement may be made in a few minutes.However, generally the anode current density should not be so high thatthe anode efficiency is below Sometimes it is possible to operate athigher current densities if the results are reproducible and a standardmetal coating is used for calibration. To avoid too frequent adjustmentof the current, the cathode current density should also be kept fairlylow. `It is quite often advantageous to have the cathode areaconsiderably larger than the anode area.

By way of example, the thickness of a thin tin electroplate on steel maybe determined in about two minutes with an accuracy of about 2%. In thiscase, it is convenient to use, as the electrolyte, an aqueous sodiumhydroxide solution of about 10% strength. However, other basic solutionsmay also be used.

Tarnished brass or copper makes an excellent cathode. Brass or-coppercan be given a tarnish film, probably copper oxide or copper hydroxide,by dipping the cathode in the electrolyte, and, While wet, exposing itto the air for several minutes until a dark film is formed.

In this type of stripping cell, using an aqueous sodium hydroxidesolution as the electrolyte and a tarnished brass-or copper cathode, thetin is dissolved according to Faradays law as the stannite at a currentdensity of about 5-10 amperes per square foot.. In order to increase theconstancy of the current,an oxidizing agent such as hydrogen peroxide,sodium hypochlorite and the like maybe added in small amounts. Forinstance, a few drops of hydrogen peroxide-solu-- tion may be added nearthe cathode afterthe cell has started to operate. However, if theperoxide is added to the electrolyte before the cell is externallyconnected, it will polarize the tin surface and make the cellinoperative. The endpoint of the measurement is obtained when thecurrent rapidly falls to near Zero. This corresponds to the completeremoval of the tin. In some arrangements of the cell, the removal of thelast traces of the tin can be observed visually.

As indicated in Figure 3, the surface of the metal coating I2 isgradually dissolved away -by anodic attack of the electrolyte Iupon themetal of said coating, `the extent of this anodic attack being limited,as indicated at 3l, to the area of the surface of the coating I2 denedby the sealing ring orwasher I9. Eventually, the coating I2 is eatenaway down to the'base metal I I, at which time the current falls to ornear zero.

`A stripping cell such as above described 'can alsobe used fordetermining the-thickness @itin 'statines-en Copper, nickel aridjetbermetals of .Zleelceatines en'steel; irenland ether metals. and (ifeadneiumeoatinss on Stee1..lreri and .other metals lt is usually`advantageous te select el.eetrelyte in which the base `metal isv.insoluble Substantially so., thus giving e ,Sharp end-point .ln that.ease. there is a ,rapid decrease-in thevelt: ese 'et the` .Stripping.eel1 kat theendeppnt and. a corresponding decrease in the omtrent.` Itvis also possible to use acid solutions, with or without inhibitors, andcertain salt solutions for electrolytes. VBy a suitable choice ofcathode material and electrolyte, measurements can be made for manycombinations of coatings and base metals. For themeasurement yof zincand lead coatings, a aqueous caustic soda solution can be used, the sameas flqrltin.

The thickness of the portions of the metal Ycoating unalloyed or alloyedwith the basemetal `,may be ascertained by the method an A appa- .alloylayers vare involved, and ,there are many' other possible combinationswhich could be treated likewise. In many cases it is possibleby lthismeans to detect and measure alloy layerswhich are not readily `visibleley-microscopic examination. Such a technique should be very valuable instudying the interdilusionof adjacent metal layers and the eiect oftreatments such as-cleaning and etching on this diffusion.

In the above description of this invention, it has been shown that thethickness of metal coatings can be simply and rapidly measured by amethod wherein a denite area of the metal coating is made the anode of avoltaic cell, and the cell is discharged at constant current until thecoating has been removed. When the metal is dissolved at 100% anodeefficiency, the thickness of the coating can be calculated by the use ofFaradays law. In a specific example, the thickness of tin on tin platedsteel is determined by making the tin the anode in a cell with a sodiumhydroxide electrolyte and a brass or copper cathode.

In the stripping cell above described, the spring 23 is of suchdimensions and loading characteristics that about one-half the weight ofthe cylinder I4 will be impressed upon the piston-like member l1. Thisprovides a tight seal on the washer I9 and also allows good contactbetween the cylinder I4 and the metal coating l2. In actual practice, itis convenient to connect the stripping cell t3 through a foot or more ofdouble wire cord to the other instruments, such as the Variableresistance 26, milli-ammeter 25 and switch 21, mounted in a box. Thispermits the maximum exibility in the operation of the instrument.

Various modications of the construction of the stripping cell arepossible. For example, the cylinder I4 can be made of material otherthan metal, and a connection to the metal coating l2 may be made bymeans of lugs or pins projecting from the base of the cylinder. Thecentral cathode 22 may be of non-uniform cross-sectional area, as forinstance by enlarging the upper porhel' @sample cf` .Srieficconstructional Operating `cehditions.'the member 1,4 be a nsteel cLinder, two inchesirlf diameter tive inches lush. The piston-like memberllegar bethree-uuarters ci aninch-inrdiameter by :one vinch in length,and made-of Bakelite. luigi-te Qt other Elastic. The cathode tubet,ttsd' tightly .Within the .piston-like memberslflf, may-:be two andone-half inchesinelengthiby th =teenths inch outside diameter y.caminartubing. The. gasket -or washer `IJJ may be .out fron-1 threersixteenthsinch linside diameter heavywa'lled rubber tubing.,

they operation of the above cell for thintin coatings, themilli-.ammeter25 .may have alca.- pacity of two or -ve milli-.amperesThe `variable lresistance 2E usedto controlthe currentvfrom thestrippingcell may be a variable wire wound rheostnt with ama-ximumresistance of 46.0.0: ohms. When these instrumentsl are used and acurrent .of 2 .milli-.amperesdrawn, the thickness. of a ltiri coating cfabout 0.5 lb./-basebox-(435-55sqsit.) isy measured ins'about. twovminutes. For-:heavier tin coat-ings and jor thevusual zinc'coatings,it;is necessary to discharge thecellat a higher cur.- rent or reduce thearea stripped inonder toreducel thetime :required fora i-neasurement.`In some cases this will ynecessitate the use ofa cathode with moresurface area. "Thelsame re: sult. can valso ybe achieved by placing moreelec= .trolyteinthe cathode tube. l l rIhe procedure 'formaking-arn'ieasurement-y with the abone described cell.k is .as follows.TheA strip?E ping cell is placed on the sheet of tinplate over thedesired area and the top of the cathode pushed down to insure goodcontact between the gasket and the tinplate. Then with an elongatedmedicine dropper a few cubic centimeters of the electrolyte are placedin the cathode tube. The rheostat is then set to a predeterminedposition, the switch closed and a stop watch started. In order toeliminate any further need for adjusting the rheostat to obtain constantcurrent, a drop of hydrogen peroxide solution may be added to theelectrolyte in the cell. The endpoint of the measurement is determinedwhen the current drops very sharply and can no longer be held constantby the rheostat. At this point the stopwatch is stopped and the cellswitched oi. The number of seconds required multiplied by the factor forthe cell and ammeter, gives the thickness or weight of the coating,depending upon what factor is used. The electrolyte is then sucked outof the cell with the medicine dropper and the cell rinsed with water.The cell is removed from the tinplate and the tinplate and cell gasketblotted with cleansing tissue. The cell is now ready for anotherdetermination. With such a stripping cell it is possible to measure thelocal thickness for Very small and close areas and thus obtain thedistribution of coating acrosslarger areas.

It will, of course, be understood that various details of the processand apparatus may be Varied through a wide range without departing fromthe principles of this invention and it is, therefore, not the purposeto limit the patent granted hereon otherwise than necessitated by thescope of the appended claims.

fagiiias's A"lilriatweclaimis: t f Q 1. Apparatus for use indetermining' the thicknessof a-metal coating over a dissimilarfoundation metal, comprising an open-'ended hollow metal member fordirect gravity contact with i thefcoating tobe measured, said memberhaving.y an upper inturned annular shoulder, an nular"'insulating andsealing means Amovable within said member for resting against saidcoating to' define a predetermined area thereof,

resilient means between said shoulder and said Y annular-insulating andsealing means transmityting a part Yonly of the weight of said metalniem- 'ber -tofurge said insulating and sealing means against'saidcoating, a hollow electrode extending into said metal memberiand fittedinto said lannular insulating and sealing means to form the side wallsof a receptacle for an electrolyte, and external electrical connectionsto said electrode as the cathode and said metal member as 1.26

the anode to complete an electrical circuit including said electrode,metal member and said electrolyte to effect an anodic dissolution ofmetal from said coating over'said predetermined area, i 2. A voltaicstripping cell for use in determining the thickness of a metal coatingover a dissimilar foundation metal, comprising a metal member for directgravity contact with the coating to be measured and having a verticallyextending recess, annular insulating and sealing means movable withinsaid recess for contact with saidA coating to conne an area thereof, ahollow'cathode fitted into said annular insulating and sealing means andmovable therewith to form thev side walls of a receptacle for anelectrolyte and insulated by said insulating and sealing means from saidmetal member and from said coating, cushioning means cooperating withsaid member and with said insuiatin'gafid sealing means and transmittinga part only Voi? the weight of said member to urge the said insulatingand sealing means tightly against said coating, and external electricalconnections between said cathode and said metal member as kthe anode tocomplete an electrical circuit includingl said cathode, anode andelectrolyte to effect a stripping of metal from said coating over saidconned area.

- WILLIAM S. HERBERT. HOWARD` T. FRANCIS.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Journal of Applied Physics,October 1939, pp. 726 through 727.

Metallurgia, December 1943, pp. 101 and 102.

Electrochemistry, by W. A Koehler (1944), second edition, p. 53.

